The present invention relates essentially to a method and an apparatus for heat treatment capable of being used for making a hydraulic binder, including a post-combustion step.
Up to the present time, the recent methods and apparatuses for making, for example, a hydraulic binder, include a previous step of precalcining of a powder material suitable for making a hydraulic binder, which may be a cement clinker, in a precalciner, and a step of calcining or burning proper in a calcining or burning kiln, which may be a clinkering kiln. The precalcining is usually performed as illustrated in FIG. 1 of the appended drawings by suspending the powder material by means of various gases, in particular by means of gases proceeding from the calcining kiln, as well as, advantageously, the hot air recovered from the cooler.
The powder material may contain a component capable of producing an exothermic reaction either with a gas or with another component. However, particularly in the case of cement clinker making, when the basic powder material does not contain such a component capable of causing an exothermic reaction, usually referred to as the "combustible", it must be mixed with such a combustible component.
Now the present tendency is towards the use of cheap combustibles. Of course, such inexpensive combustibles do not have the same properties, particularly, of reactivity and of aptitude to grinding, as the combustibles of the higher qualities.
As is well known in the art, the poorer the quality of the raw material or the combustible used, the more difficult it is to correctly carry out the method of making the hydraulic binder, particularly the cement clinker. Moreover, it is generally recognized by those conversant with the art that the presently known apparatuses must be extensively modified to ensure a correct making of the hydraulic binder, in particular consisting of a cement clinker.
Indeed, as is well known, the previous step of precalcining constitutes an essential step in the preparation of the raw material before the heat treatment for the making of the hydraulic binder in the calcining or burning zone. It is also readily understood that such previous combustion is much more difficult to achieve when the initial combustible is of poor quality and therefore more economical. Among such initial combustibles are for example the poorest coals, the charcoals, the petroleum cokes, etc.
Of course, the combustion during the precalcining step could be improved by increasing the time of presence in the precalciner.
The time of presence of the gases in the precalciner reflects the time of presence of the material and of the combustible.
Now this time of presence of the gases is directly related to the volume of the precalciner.
If it is desired, for example, to double the time of presence, the volume of the precalciner must be at least doubled.
Thus, for example, in an apparatus for the production of 2,000 tons of clinker per day, the volume of the precalciner is 160 m.sup.3 and the doubling of such an apparatus is impracticable from the technological point of view.